The present invention relates to a bath composition for the electropolishing of a metal surface made of nonalloyed titanium, and to a method of using this bath.
The term xe2x80x9cpolishingxe2x80x9d is understood to mean a treatment for the purpose of reducing the roughness of a metal surface and, thereby, of increasing its brightness with, as a consequence, a lower corrosion sensitivity.
Apart from the mechanical means used for this purpose (the use of abrasive powders of decreasing particle size, fine machining, honing, etc.), there are also techniques which rely on the use of chemical and/or electrolytic reactions. Thus, the technique is named xe2x80x9cchemical polishingxe2x80x9d when the reactions generated do not rely on an external current source and the technique named xe2x80x9celectropolishingxe2x80x9d when the reactions depend on an external current source, one of the electrodes (generally that connected to the positive pole of the electric current source) consisting of the workpiece to be polished.
The present invention falls within the technical context of electropolishing.
Electropolishing relies on two simultaneous and opposing reactions in which the relative rates and the diffusion phenomena at the metal/solution interface control the operating process. One of these reactions is a dissolution reaction during which the metal passes into solution in the form of ions; the other reaction is an oxidation reaction during which an oxide layer forms, this layer being protective to a greater or lesser extent and limiting, when it is present, the progress of the first reaction. These two, opposing and complex, reactions come into competition with the consequence that the chemical etching undergone by the metal surface is self-limiting, polishing being merely one particular result of this etching.
The polishing obtained electrolytically is substantially influenced by the viscosity and/or resistivity of the electrolyte employed. It is known to use various acid compositions, especially compositions based on hydrofluoric, sulfuric, nitric and phosphoric acids in various concentrations. Some of these acids (for example hydrofluoric acid) allow the oxide layer formed on the metal surface to be dissolved, whereas other acids (for example phosphoric acid, sulfuric acid, etc.) form the viscous medium necessary for the electropolishing to progress. It is essential for the concentrations of the constituents of the electrolytes to be correctly controlled in order to ensure that the process progresses properly and to determine the lifetime of these electrolytes.
Many electropolishing bath compositions are known (see for example U.S. Pat. Nos. 3,766,030, 3,864,238, 5,591,320, 5,565,084, etc.). Some of these known compositions are multipurpose and allow both pure titanium and its alloys to be treated. Because of this, the quality of action of these baths is the result of a compromise and polishing of the metal surfaces treated is not optimal.
The object of the present invention is therefore essentially to provide a bath composition for the specific electropolishing of nonalloyed titanium so as to obtain a metal surface having a high-quality and measurable degree of polishing, but also so as to obtain, by an appropriate choice of the electrical parameters when using the composition, metal surfaces having a roughness which is predeterminable (xe2x80x9cadjustablexe2x80x9d) and measurable (for example in the case of biocompatible body implants made of titanium).